Dust particle

“ Dust-wells.—The wind-blown dust sometimes gives rise to peculiar topographic features of small size. The dust is not distributed by the270 wind with absolute equality, and the surface drainage of the ice tends to aggregate it. Every dust particle acts like a small stone, and where aggregations of dust occur, they melt their way down into the ice, developing holes or “dust-wells” (Fig. 245) . These wells rarely reach a depth of more than a few inches, but they may be so numerous that the pedestrian is obliged to watch his steps. This is especially true near the edge of the large ice-caps. ”

“ The surface is almost infinitely great in comparison. Now, the electric charge of a dust particle, condensed on its surface, is of the same kind as that of the earth. Therefore they mutually repel. It is only fair to add that the theory of the electrification of dust is not fully substantiated. ”

“ It is evident that if there were not some purifying process in nature there would be a tendency for the dust particles to increase in numbers, because though some dust particles may fall out of the air, many of them are so small they have but little tendency to settle, but by becoming centres of cloud particles they are carried downwards to the earth, and, further, these when showering down as rain tend to wash the others out of the atmosphere. ”

“ With this we come to the most important function of dust in our atmosphere—the part which it has in the function of rain by reason of vapors condensing upon it. It can be affirmed with certainty that all the water which the sun causes to evaporate on the surface of the sea and on the land is condensed again on dust, and that no raindrop falls unless it had a particle of dust as its primary nucleus.
When we speak of "vapor" we always mean water in the gaseous condition, transparent and invisible, like all other gases but cloudy steam, such as is seen escaping from the boiler of a locomotive. ”

“ John Aitkin, the highest authority on the subject, has shown that clean air contains from 3000 to 5000 visible dust particles per cubic inch. The air of schoolrooms and public buildings with undressed wood floors carries from 60,000 to 80,000 particles which are visible under the high power of a microscope, and an unknown number which can be counted only when amplified in size by the condensation of moisture upon their surface. ”

“ The numbers of dust particles found by different observers in a single cubic centimeter of air have varied from 157 at the summit of the Swiss Bieshorn to more than 200,000 in a Parisian garden. On dusty streets and within doors, especially in dusty trades, still more dust may exist, and it is estimated that a single puff of tobacco smoke discharges into the atmosphere 4,000,000,000 particles. Dust may consist of inorganic and lifeless organic matter, as well as bacteria and other living organisms. ”

“ It could be evaporated and it would spread and diffuse in the air and, as it spread, here an atom and there an atom would break down, emitting radiation and becoming another substance entirely.
"And that would be a new compound which wouldn't stay vapor but would come out as a microscopic particle of dust with an electric charge that would draw moisture or other particles to it! It would grow and grow and ultimately settle down as a dust-mote too small to be seen.
"And that would happen quintillions and quintillions and quintillions of times, and motes of poison would settle—are settling...." ”

“ But if dust makes the sky appear clear, why is the color of the sky blue? Why does dust, of the different constituents of white sunlight, reflect the blue rather than the green, yellow, and red? This fact is connected with the size of the dust particles. Only the finest dust settles so slowly that it can be spread everywhere by means of the air currents, and can be found constantly in all strata of the atmosphere ”

“ These dust particles, when air is at rest, constantly fall to the ground under the action of gravity, and are deposited on shelves and ledges, from which they have to be removed daily by the housemaid. When air is warmed and ascends it carries the dust particles with it, and these particles striking against any cold surface with which they come into contact stick to it. ”

“ Aqueous vapour or mist, suspended in the atmosphere, becomes visible exactly as dust does in the air of a room. In the shadows, you not only cannot see the dust itself, because unillumined, but you can see other objects through the dust, without obscurity; the air being thus actually rendered more transparent by a deprivation of light. Where a sunbeam enters, every particle of dust becomes visible, and a palpable interruption to the sight; so that a transverse sunbeam is a real obstacle to the vision—you cannot see things clearly through it. ”

“ Many of the dust particles in the atmosphere which form the nuclei of condensation are extremely minute, so small as to be beyond the powers of the microscope, and at first sight it might appear to be impossible to get any reliable information as to their numbers. But Aitken, having shown that water vapour must have a nucleus to condense on, saw that this placed in our hands the means of counting the dust particles in our atmosphere, and in 1888 showed how it could be done. ”

“ Hence when the atmosphere is heavily charged with dust particles that have become moisture laden, as they will in a humid atmosphere, and therefore relatively bulky, we see the sun as a fiery red ball. We know, too, that this dust has much to do with rainfall for, as was first proved many years ago by the physicist Aitken, cloud particles, and, therefore, rain, will not, under ordinary conditions, form in a perfectly dust-free atmosphere, but will readily form about dust motes of any kind in an atmosphere that is sufficiently damp. ”

“ John Aitken, F.R.S., of Falkirk, Stirlingshire, contrived a means of gauging the dust contents of the atmosphere. This consists of a glass box about a centimetre in thickness. Two pieces of wet filter paper inside serve to keep the contained air damp. The bottom of the box is a micrometer plate, divided rectangularly in millimetres. It can be examined from above by a lens. An air-pump can withdraw definite volumes of air as desired. When the air is partially withdrawn, the expansion of the remainder produces cooling. The dust particles form nuclei for condensation of the vapour. ”

“ If, then, air, which always has dust particles in it, containing all or nearly all the water vapor it can hold, is cooled to a distinctly lower temperature, a corresponding amount of condensation will take place on each dust mote, and the countless droplets thus formed will appear as a fog or cloud of greater or less density. ”

“ Without atmospheric dust, we should not have the glorious cloud scenery which we at present enjoy. We should have no haze in the atmosphere, we should have no twilight. Darkness would come as soon as the sun passed below the horizon.
The relative humidity of the air has a great effect on the dust by increasing the size of the particles of water vapor and so increasing the haze. The number of dust particles rapidly decreases with the amount of moisture present. ”

“ Some of the vapor of the air, on condensing, gathers on solid objects like grass, or glass; but some, as fog, floats about in the air. Really this, too, is often gathered around solid objects. Floating about in the air are innumerable bits of "dust" which you can see dancing about in the sunlight when a sunbeam enters a dark room. Some of these "dust" particles are actual dust from the road, but much of it is something else, as the pollen of plants, microbes, and the solid bits produced by the burning of wood or coal. ”

“ We do not usually realize that, though the air itself in inhabited rooms is constantly changing more or less rapidly by diffusion, by draughts, or by purposed ventilation, fine dust particles are not removed under the same influences in proportionate degree. They cling more or less tenaciously to all surfaces on which they have settled, and especially to fabrics, so that currents of abundant force and sufficient distribution to change the air may and usually do leave the lodged dust particles almost entirely undisturbed. ”

“ Condensation does not take place so readily—in other words, the dust particles are indifferent nuclei. A thick fog condensed on nuclei of an indifferent sort may be “eaten up” by a slight rise in temperature; it may rain itself to pieces by a drop in temperature.
Sources of Atmospheric Dust.—Aside from the dust picked up and carried by the winds, there are well-defined sources of floating dust that must be considered. Cosmic, or meteoric dust, is not born of the earth; it is gathered by the earth from space. Large particles fall to the earth ”

“ Turning, now, our attention to the atmosphere, we find that at nearly all times, and everywhere within two miles of the surface, and probably much higher still, it contains, in every cubic inch, thousands of dust particles coming from fires, from plants, from the dry earth as caught up by winds, and from still other sources. Much of this dust is excessively fine and settles down with extreme slowness. It serves, as already explained, as nuclei about which the myriads of cloud droplets are formed. ”

“ As these microorganisms do not exist in the air as isolated units, but travel upon small or large dust particles in the case under consideration, the carrying medium most probably is the fine impalpable dust from which it is practically impossible to free the air of a building. ”

“ This fine dust was carried completely around the earth and from the extreme north to the extreme south of the largest continents. Moreover, it continued to exist for several years as a component of the earth's atmosphere. To dust particles we owe the existence of clouds, fog and haze, the beautiful colors of the sunset, and in large part the blueness of the sky. Dust is thus our constant companion and with every breath we inhale much of it. ”

“ The particles themselves are too small to be discerned by any mechanical method at present known. En masse they reflect enough sunlight to reveal their existence, but not their form nor their constitution. In part, and probably to a great extent, they constitute the overhead effect noted by observers for more than six thousand years—the sky. An estimate of the size of such dust particles cannot be made with any degree of accuracy. It is safe to say that they are much smaller than the smoke particles that escape from the burning end of a cigar. ”

“ Usually before one hundred strokes a larger volume of smoke tells that the dry dust constantly falling on the pile has grown more and more charred until finally a tiny friction-fired particle falls, carrying combustion to the already heated dust cone. ”

“ However, supposing there is any part of the upper air free from dust, it is now found possible, when any of these gases experimented on be present, for the sun to convert them into nuclei of condensation, and permit of clouds being formed in dustless air, miles above the surface of the earth.
In the lower atmosphere there are always plenty of dust-particles to form cloudy condensation, whether the sun shines or not. These are produced by the waste from the millions of meteors that daily fall into the air. ”

“ As in the case of microbes, the number of dust particles is far greater in cities than in the country, being least on high mountain tops and over the oceans. The air in large cities invariably shows hundreds of thousands of dust motes to the cubic centimeter, that of the village thousands, and that of the open country some hundreds. Dust-free air is also germ-free. Many experiments have shown that air freed of dust motes has at the same time been cleared of the microörganisms that cause disease, putrefaction, and fermentation ”

“ There can be no doubt, therefore, that dust forms a very important distributing agent for micro-organisms, dust particles, aided by the wind, being to bacteria what the modern motor-car, with its benzine or electric current, is to the ambitious itinerant of the present day. ”

“ Sulphur dioxide is a gas and is not to be included in the dust content of the air. But the intense heat of combustion has separated it from the combination in which it existed. The chemical affinity of the nascent gas is strong, and in the air it is apt to enter into combination again with dust particles of basic character, the resulting combination forming nuclei favorable to rapid condensation. ↑ Volcanic “ash” is not a product of combustion. It is the convenient name applied to lava blown into fine dust by the expansive force of steam, or by other forces. ”

“ How will he account for the absolute immunity of a liquid exposed for months in a warm room to optically pure air, and its infallible putrefaction in a few days when exposed to dust-laden air? He must, I submit, bow to the conclusion that the dust-particles are the cause of putrefactive life. And unless he accepts the hypothesis that these particles, being dead in the air, are in the liquid miraculously kindled into living things, he must conclude that the life we have observed springs from germs or organisms diffused through the atmosphere. ”

“ These dust-particles are not the motes that we see in the path of a sunbeam; for, when common air is passed through a flame, these motes disappear, but the air still remains a good medium for fogs. It is a finer kind of dust which furnishes the fog and cloud nuclei. The products of combustion are fog producers, and especially the vapor from the burning of sulphur. ”